Hydroxyl acrylic resins, incorporating

Hydroxyl Acrylic Resins [13] AJlyl alcohol and allyl alkoxylate have l n incorporated into hydroxyl acrylic resins to provide the pendant hydroxyl groups for crosslinking reaction. The resins are crosslinked with melamine or an isocyanate to form acrylic-melamine coatings or acrylic-polyurethane coatings, respectively. The coatings can be used as automotive topcoat or other high performance industrial coatings. 

Polyacrylates as binders consist of copolymers of acrylate and methacrylate esters. Other unsaturated monomers (e.g., styrene and vinyltoluene) may also be incorporated, but usually to a lesser extent. Copolymers formed exclusively from acrylates and/or methacrylates are termed straight acrylics. The comonomers differ as regards the alcohol residues of the ester group, which also allow incorporation of additional functional groups. Choice of suitable monomers allows wide variation of the physical and chemical properties of the resulting polymer. Hydrophilicity, hydrophobic-ity, acid base properties as well as can be adjusted resins containing hydroxyl, amine, epoxy, or isocyanate groups can also be produced. 

Thermosetting acrylics are produced by incorporating functional carboxyl, hydroxyl, or amide groups during free radical polymerization. Acrylamide and acrylic acid are often used as functional monomers for this purpose. The thermosetting acrylics are formulated with other reactive resins, such as epoxies, to provide a crosslinked film on heating. These films are considerably harder, tougher and more chemically resistant than those attainable from thermoplastic acrylics. 

The majority of water reducible resins prepared by the solution route are for thermoset applications if they are to be used as the principle film forming resin. As such hydroxyl functionality is normally incorporated if the resin contains acid functionality for neutralisation, which is normal. Typically, the hydroxyl value would be of a similar value to the acid value, i.e. 50 - 100 mg KOH/g. Note that there are two methods of quoting acid and hydroxyl values for vinyl and acrylic solution resins. Values can be quoted... 

Melamine formaldehyde can be crosslinked at elevated temperatures with both hydroxyl and carboxyl functional groups. (See thermosetting acrylics chapter for reaction mechanisms.) The temperature required is at least 120°C, at which point the hydroxyl group will react, but the carboxyl group needs a slightly higher heat input, approximately 150°C. Systems are unlikely to require an acid catalyst because of the catalytic effects of the polymerisation catalysts and surfactants in the acrylic latices. If required, p-toluene sulphonic acid is the most suitable (typically at levels of 0.2 - 0.4%). Alternatively, the melamine resin could be incorporated in an unneutralised, acidic emulsion, which reduces the cure temperature, but will sacrifice stability. 

Thermosetting acrylic binder systems utilize copolymers of functional and nonfunctional acrylic (or similar) monomers. The functional monomers are incorporated for reactivity with crosslinkers. The most common functional monomer for reactions is the hydroxyl group. The hydroxyl groups on the acrylic copolymers react with melamine and urea resins (amino resins) and with polyisocyanates. These reactions are shown in Figure 11. The reaction of hydroxy functional polymers with amino resins require acid catalysis and heat. The reaction with polyisocyanates can occur at room temperature as well as at higher temperatures. A number of materials will catalyze the hydroxyl/isocyanate reaction (organotin compounds, acids, amines, metal salts, etc.)(9). 

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